1. Field of the Invention
The present invention relates to an optical pickup actuator, an optical pickup, and an optical disc drive apparatus, and more particularly, to an optical pickup actuator blocking leakage magnetic flux, an optical pickup employing the actuator, and an optical disc drive apparatus employing the pickup.
2. Description of the Related Art
An optical pickup actuator is used in an optical recording apparatus for recording and reproducing data on an optical disc. As optical recording apparatuses become smaller and compact disc access time becomes shorter, an optical pickup actuator having a lower height and a faster follow-up speed is desired. Also, new optical pickup actuators should be designed to allow use of existing optical discs such as CDs and DVDs. Such an optical pickup actuator should have the same, or wider, operation range as existing optical pickup actuators to satisfy the aforementioned requirement.
Leakage magnetic flux generated in a magnet, installed in an optical pickup actuator, interacts with current to produce a nonuniform thrust force in the optical pickup actuator. This nonuniform thrust force causes negative resonance in the optical pickup actuator which increases as the access time of the optical pickup actuator decreases. As defined herein, negative resonance means an unstable operation of an optical disc caused by resonance at natural frequencies of the optical disc and the optical pickup actuator.
FIG. 1 is an exploded perspective view of a conventional asymmetric optical pickup actuator. Referring to FIG. 1, the conventional optical pickup actuator includes first and second magnets 11a and 11b, yokes 13a and 13b in which the first and second magnets 11a and 11b are fixed, and a base 15 on which the yokes 13a and 13b are installed. A focusing coil 12 and tracking coils 16 interact with the first and second magnets 11a and 11b and generate electromagnetic force. A blade 17 is provided so that the focusing coil 12 and the tracking coils 16 interact with the first and second magnets 11a and 11b. An objective lens 19 is seated on the blade 17, and a suspension 18 supports the blade 17.
FIG. 2 shows magnetic field lines around the first and second magnets 11a and 11b and the focusing coil 12 of the optical pickup actuator shown in FIG. 1.
Focusing and tracking operations of the optical pickup are performed by an electromagnetic force F resulting from an interaction of the first and second magnets 11a and 11b, installed in the yokes 13a and 13b, with the focusing coil 12 and the tracking coils 16, installed on the blade 17. The electromagnetic force is given by Equation 1 below:{right arrow over (F)}={right arrow over (IL)}×{right arrow over (B)}  (1){right arrow over (F)} is the electromagnetic force vector measured in Newtons (N), {right arrow over (I)} is the vector current measured in Amperes (A), {right arrow over (L)} is the length of a section of the coil affected by the magnetic field measured in meters (m), and {right arrow over (B)} is a magnetic field vector in the region of the section of coil measured in Tesla (T).
The optical pickup actuator performs a focusing operation to read or record data on an optical disc and operates within an operational range so as to follow-up the optical disc when the optical disc is disturbed. During the follow-up operation, the optical pickup actuator moves upward and downward with respect to a plane of the base 15. The electromagnetic force that acts on the focusing coil 12 and the blade 17 is generated by a magnetic field B and current I due to current flowing through a coil positioned between the first and second magnets 11a and 11b, as shown in Equation 1. In order to drive the optical pickup actuator in a focusing direction, it is preferable that magnetic flux only exists between the first and second magnets 11a and 11b. However, in conventional actuators, magnetic flux also leaks behind the second magnet 11b, thereby causing negative resonance.
Such leakage magnetic flux generated from an N pole of the second magnet 11b shown in FIG. 2 interacts with current flowing through a section of the focusing coil 12 placed behind the second magnet 11b with respect to the objective lens. Due to this leakage magnetic flux, a nonuniform electromagnetic force is generated in the focusing direction, thereby causing negative resonance in the optical pickup actuator. Due to the negative resonance in the optical pickup actuator, the optical pickup operates unstably, and the overall performance of an optical disc drive apparatus is lowered.